Project Summary Major depressive disorder (MDD) is the leading cause of disease burden worldwide and women have a 2-fold risk higher than men. Despite this, little is known about the neurobiological mechanisms associated with the sex-dependent emergence of MDD and in what form they persist after full remission. Preclinical studies have demonstrated dysregulated hypothalamic pituitary adrenal (HPA) circuitry (i.e., stress circuitry) and gamma- aminobutyric acid (GABA) in depression models. Critically, the stress circuitry houses some of the most highly sexually dimorphic regions in the brain. Using an innovative multi-modal imaging approach, the proposed study will investigate the role of GABA (assessed by proton magnetic resonance spectroscopy; MRS), functional brain activity deficits within key stress circuitry regions, as well as gonadal and adrenal hormones in understanding sex differences in MDD. Further, we will link these abnormalities to fundamental dimensions of behaviors (anhedonia and stress sensitivity) that capture key MDD vulnerabilities. Goals are to investigate: (1) functional and neurochemical features of sexually dimorphic regions within the stress circuitry irrespective of clinical state; (2) moderating effects of steroid hormonal dysfunction on stress circuitry in MDD; and (3) sex differences in anhedonic behavior and inability to regulate arousal under stress. To this end, 144 unmedicated young adults ages of 18-25, 96 with current or remitted MDD (rMDD), will be recruited, equally divided by sex. Capitalizing on a novel triple-voxel MRS technique affording improved temporal and spatial resolution, GABA transmission will be simultaneously assessed from three sexually dimorphic regions implicated in stress regulation (rostral anterior cingulate cortex (rACC), medial prefrontal cortex (mPFC), and hippocampus (HIPP)). In addition, functional MRI will be collected during a well-established stress paradigm. Participants will also undergo a probabilistic reward task under ?no-stress? and ?stressful? conditions. Gonadal and adrenal hormones will be collected during baseline and stress/reward tasks to relate sex-dependent hormonal abnormalities to task performance and imaging data. Based on our extensive preliminary data, we hypothesize that: 1) Relative to healthy women, women with MDD will show dysregulated (1a) activation and (1b) functional connectivity within the stress circuitry, and (1c) lower GABA signaling in HIPP, mPFC, and rACC. Further, we expect that (1d) predicted sex differences will emerge irrespective of current vs. remitted MDD status. (2) These neural abnormalities will be associated with physiological deficits in steroid hormone levels; and (3) Reduced GABA and lower activity in HIPP, mPFC, and rACC will predict greater stress-induced anhedonia, particularly among MDD women. Using categorical and dimensional analytic approaches, the proposed integration of laboratory-based measures of reward and stress sensitivity, state-of-the-art imaging techniques, and hormonal assessments promises to provide novel insights in the sex-dependent manifestation and pathophysiology of MDD.